The present invention relates to an ignition system for an internal combustion engine.
Ignition systems act to ignite a compressed fuel-air mixture in the internal combustion engine. For this purpose, using an ignition device, usually a spark plug, an arc discharge is generated between two electrodes of the spark plug. To generate this arc discharge, an ignition voltage in the high-voltage range is made available. To make this necessary high voltage available, a spark plug may be connected to the secondary winding of an ignition coil which has a primary winding that can be connected to a voltage source, which in motor vehicles is usually the motor vehicle battery. In this context, the ignition coil operates as an energy storage device and as a transformer. During the closing time of the primary-side switching means, the electrical energy made available from the voltage source is stored in the magnetic field of the ignition coil, and, at the ignition time point, it is made available as a high-voltage ignition pulse.
To ignite the compressed fuel-air mixture, a specific minimum ignition energy is necessary. The level of this minimum ignition energy is a function of the stochiometric composition of the fuel-air mixture. In particular, when the fuel-air mixture is lean, i.e., air is present in stochiometric excess, then an increased minimum ignition energy is necessary. If this minimum ignition energy is not made available, the result can be the incomplete combustion of the fuel-air mixture or ignition misfiring. The conventional options for influencing the combustion process are in varying the spark duration and/or the spark current. To increase the spark duration and/or the spark current, it is conventional to increase the energy that is stored on the primary side of the ignition coil, for example, by increasing the primary current on the primary side. In this context, however, a disadvantage arises where it is necessary to select a correspondingly large design of an ignition coil. The large design of the ignition coil hampers the goal of optimizing the overall installation volume.
The ignition system according to the present invention may provide high ignition energy, which may be adequately proportioned in every operating situation of the internal combustion engine, especially in igniting lean fuel-air mixtures. As a result of the fact that two ignition coils are provided, each having a secondary winding that is connected to one spark plug and with primary windings that may each be acted upon by a switching arrangement using the supply voltage, and that a drive circuit is provided which allows a time-displaced driving of the switching arrangement and therefore of the ignition coils, it may be possible to switch on the second ignition coil precisely at the time point at which, in the voltage circuit of the first ignition coil, the switch-off voltage results in the secondary-side generation of the high voltage. In this manner, on the high-voltage side of the second ignition coil, a positive switch-on voltage arises, which is added to the negative spark voltage of the ignition spark generated by the first ignition coil, and therefore the spark voltage present at the ignition electrodes of the spark plug is increased, specifically, more than doubled. In this manner, a greater ignition spark duration and a higher ignition spark current are obtained, resulting generally in making available greater ignition energy. This high ignition energy may be well-suited to reliably igniting even lean fuel-air mixtures every time. By connecting in alternating fashion the other ignition coil in the switch-off phase of the previously connected ignition coil, it is possible repeatedly to extend the spark duration over a longer time period.
The present invention is discussed in greater detail below in exemplary embodiments on the basis of the attached drawings.